1. Field of the Invention
The present invention relates to a thermoelectric conversion module for use in an apparatus utilizing a thermoelectric effect such as electronic cooling apparatus and electric power generating apparatus, and more particularly to a thermoelectric conversion module having N type semiconductor elements and P type semiconductor element connected in cascade by means of metal electrodes. The present invention also relates to a method of manufacturing such a thermoelectric conversion module.
2. Related Art Statement
There have been proposed various kinds of thermoelectric conversion modules utilizing the Seebeck effect, Peltier effect and Thomson effect. Among these thermoelectric conversion modules, there have been realized a Seebeck effect element and Peltier effect element, in which a thermoelectric element is formed by joining different kinds of metals. In the Seebeck effect element, different kinds of metals are joined to constitute a closed loop, and thermoelectricity is generated by making junctions at a different temperatures. Such a Seebeck effect element may be utilized as thermoelectricity element. In the Peltier effect element, different kinds of metals are joined to form a closed loop and an electric current is passed through the loop in a given direction to effect heat absorption at one junction point and heat generation at the other junction point. Such a thermoelectric element may be utilized as a thermoelectric heating element or thermoelectric cooling element. In order to improve the efficiency of these elements, a junction between a semiconductor and a metal has been widely used, because a larger Seebeck coefficient and Peltier coefficient can be obtained by a semiconductor-metal junction than a metal-metal junction.
FIG. 1 is a schematic view showing a principal structure of a known thermoelectric conversion module constructed as the above mentioned thermoelectricity element. The thermoelectric conversion module comprises a number of N type semiconductor elements 1 and a number of P type semiconductor elements 2, said N and P type semiconductor elements being arranged alternately. Adjacent N type and P type semiconductor elements 1 and 2 are connected in cascade by means of electrodes 3 made by metal segments. The left side N type semiconductor element 1 and the right side P type semiconductor element 2 of the cascade connection semiconductor element array are connected to opposite ends of a load 4. One side of the semiconductor array is placed in a higher temperature environment and the other side is placed in a lower temperature environment. Then, in each of the N type semiconductor elements 1, electrons flow from the high temperature side to the low temperature side as shown by solid lines (an electric current flows from the low temperature side to the high temperature side). In each of the P type semiconductor elements 2, holes flow from the high temperature side to the low temperature side as depicted by broken lines (an electric current flows from the high temperature side to the low temperature side). Therefore, a voltage is applied across the load 4 with a polarity as depicted in FIG. 1. The semiconductor elements 1 and 2 may be made of Bi--Te semiconductor (for instance Bi.sub.2 Te.sub.3), Bi--Sb semiconductor (for example Bi.sub.0.88 Sb.sub.0.12) or Si--Ge (for instance Si.sub.0.8 Ge.sub.0.2).
FIG. 2 is a perspective view showing a known method of manufacturing the above mentioned known thermoelectric conversion module. On a surface of an insulating substrate 5 are secured electrode metal segments 6 by brazing in accordance with a given pattern. Then, N type semiconductor elements 1 and P type semiconductor elements 2 are secured to the metal strips 6 by brazing or soldering. The semiconductor elements 1 and 2 may be formed by a single crystal melting method or a sintered semiconductor material cutting method. On upper surfaces of the N type and P type semiconductor elements 1 and 2 there are secured metal segments 7 by means of brazing or soldering. In this manner, the N type semiconductor elements 1 and P type semiconductor elements 2 are arranged alternately and are connected in cascade by means of the metal strips 6 and 7. In this case, it has been proposed to secure the metal segments 7 simultaneously to the semiconductor elements 1 and 2 by using an insulating plate on which a metal electrode pattern is previously formed.
In Japanese Patent Publications Nos. 58-199578 (JP 58-199578), 61-263176 (JP 61-263176), 5-283753 (JP 5-283753), 7-162039 (JP 7-162039) and 8-18109 (JP 8-18109), there are disclosed various known methods of manufacturing thermoelectric conversion modules. In JP 58-199578, after N type semiconductor elements and P type semiconductor elements are arranged alternately, spaces between adjacent semiconductor elements are filled with an adhesive agent. In JP 61-263176, there is described a method, in which an N type semiconductor layer and a P type semiconductor layer are successively deposited one on the other, spaces other than contact regions of these layers are filled with a glassy material. In a method disclosed in JP 5-283753, N type semiconductor elements and P type semiconductor elements are alternately arranged in multi-holes of a heat resisting isolator. Further, in JP 7-162039, there is described a method, in which a single array of through holes are formed in a mold body and N type semiconductor elements and P type semiconductor elements are alternately inserted in these through holes. Finally, in JP 8-18109, there is disclosed a thermoelectric module comprising N type and P type semiconductor elements and an insulating material such as synthetic resin, ceramics and glass filling spaces between adjacent semiconductor elements. Such a thermoelectric module is manufactured by forming an N type semiconductor layer on a glass substrate, forming a P type semiconductor layer on the other glass substrate, cutting the semiconductor layers by a dicing machine, respectively to obtain members in which pillar-like N type and P type semiconductor elements are aligned, assembling these members such that the N type semiconductor elements and P type semiconductor elements are arranged alternately, and filling spaces between these semiconductor elements with an insulating material.
When a large capacity thermoelectric conversion module including a large number of thermoelectric elements is to be manufactured by the known method shown in FIGS. 2, extremely high working precision and high assembling faculty are required, and thus a manufacturing cost will be increased very much. Moreover, it is impossible to manufacture a thermoelectric conversion module having a curved surface. Such a curved surface is required when a thermoelectric conversion module is secured to a base member having a curved surface. In this manner, the module made by this known method could not be used in various applications. For instance, when the thermoelectric conversion module is applied to a system in which an electric power is generated by using a wasted heat of an internal combustion engine, a space for providing the thermoelectric conversion module is limited and in many cases it is desired to provide the thermoelectric conversion module on a curved surface. However, the module made by the above mentioned known method could not have a curved surface, and therefore could not be applied to such a thermoelectric power system.
In the known method described in JP 58-199578, the arrangement of the N type and P type semiconductor elements requires very complicated work, high working precision and high assembling faculty, and thus a manufacturing cost becomes very high. In the known method described in JP 61-263176, due to a difference in a thermal expansion coefficient between the semiconductor material and the glassy material, the thermoelectric conversion module is subjected to damage through a heat cycle and has a short life time. In the known methods disclosed in JP 5-283753 and 7-162039, the insertion of the N type and P type semiconductor elements into the holes of the insulating substrate requires high working precision and faculty, so that the manufacturing cost becomes very high. Furthermore, the thermoelectric conversion module might be damaged through a heat cycle due to a difference in thermal expansion coefficient. In the known method proposed in JP 8-18109, the array of semiconductor elements is formed by the dicing machine, it is very difficult to manufacture a thermoelectric conversion module having a small size. Therefore, a capacity of the thermoelectric conversion module is limited. Further, due to a difference in thermal expansion coefficient between the semiconductor elements and the insulating material filling the spaces between these semiconductor elements, the thermoelectric conversion module might be damaged and its durability is also limited.